Phototube



Dec. 29, 1936. T VES 2,066,081

PHOTOTUBE Filed March 13, 1935 i Fig 2 I INVENTOR. MARTEN CORNELIS TEVEE)l BY flC ATTORNEY.

Patented Dec. 29, 1936 UNITED STATES PHOTOTUBE Marten Cornelia Teves, Eindhoven,

Netherlands,

minor to Radio Corporation of America, a corporation of Delaware Application March 13, 1935, Serial No. 10,764 In Germany March 26, 1934 3 Claims. (01. 250-215) My invention relates to light sensitive electron discharge devices, more particularly to improvements in photo-cathodes'and the method of manufacturingthe same.

In the manufacture of photo-tubes having photo-electric electrodes on photo-cathodes which have a high electron emitting capacity, it is conventional practice to deposit a metal layer, usually of silver, on the inside wall of the tube envelope and to thensuperficially or entirely oxidize this layer and subject it to the action of an electropositive metal, for example, analkali or alkaline earth metal. The oxidation of the silver metal layer is usually effected by heating the metal in an atmosphere containing oxygen or by means of an electricdischarge in such an atmosphere.

It has been found that this oxidation method introduces a number of difficulties. The oxidation by electrical means for example is limited substantially to the oxidation of silver layers. When other metals, for example, copper and nickel are used, the results are often unsatisfactory. When it is desired entirely to oxidize the metal layer, and if it is deposited on an insulating base, for example, glass,-the oxidation takes place irregularly and not to the same depth throughout the metal layer. Because of this, parts of the layer which are not oxidized are separated by parts of the layer which have been completely converted into oxide, the result being that current passes thru the electrode with difficulty or not at all. While further oxidation is not impossible, it is nevertheless considerablyhampered. If the metal layer which is to be entirely transformed into a metal oxide, is applied to a metal base, the irregular oxidation of the metal layer results in a layer which is not completely oxidized or, in oxidizing the base at numerous points. This difliculty is encountered, for example, when a thin silver layer applied to a copper or nickel layer is to be oxidized. The copper or nickel oxide introduces difilculties when the electropositive or alkaline earth metal acts on the silver oxide. Such difficulties also occur when the metal layer is oxidized by heating in an atmosphere containing oxygen. 7

The object of my invention is to provide an improved method of manufacturing a photocathode having a uniform structure and which can be provided on either an insulating or conducting base.

According to my invention the metal from 'which the oxide layer is formed is supported within the tube envelope out of contact with the period of time.

base on which the oxide layer is to be formed. A discharge in an atmosphere containing oxygen is established within the discharge tube between the metal and another electrode, the metal serving as a cathode. The position of this metal as well as the current strength are so chosen that a layer of oxide of the metal deposits on the base. In this way a uniformly oxidized layer is obtained. My invention is particularly suitable for manufacturing photo-electric electrodes which 1 must be transparent to light and in which the metal oxide layer must be formed on glass or a similar material transparent to light. The electrodes manufactured according to my invention can be rendered emissive not only by light but also 15 by other means, for example, by ion orelectron bombardment causing secondary emission.

The novel features which I believe to be characteristic of my invention are set forth with particularity in the appended claims, but the inven- 20 tion itself will best be understood by reference to the following description taken in connection with the accompanying drawing in which:

Figure 1 is a vertical section of a light sensitive electron discharge device by means of which my invention may be practiced.

Figure 2 is a perspective of a light sensitive electron discharge device of difierent construction by means of which my invention may be practiced.

In Figure 1 a photo-electric tube having aglass 30 envelope l, is provided with a stem 2 to which is secured a metal rod 4 consisting, for example, of

molybdenum, and at the end of which is provided a block 5 of silver. The rod 4 is surrounded by an insulating tube 3. The stem also carries a short rod 6 which like the rod 4 is connected to a current supply wire. A conductor 1 bearing on the inner side of the wall of the tube envelope is sealed into the wall opposite the stem 2.

After the envelope i has been exhausted, a small quantity of oxygen, for example, at a pressure of about 1 mm of mercury is introduced into the envelope, and an electric discharge established between the silver block 5 (which acts as a cathode) and the rod 6 (which acts as an anode). The current strength is so chosen that the silver block 5 disintegrates and a silver oxide layer 8 deposits on the wall of the envelope. In some cases excellent results have been obtained with current intensities of a few milli-amperes for example. It has been found that the silver oxide layer is very regular and that the thickness of the layer can be regulated within fine limits by maintaining the discharge for a longer or shorter After the silver oxide layer I has been formed, the remaining oxygen is removed from the envelope and caesium or another alkali metal is introduced and madeto react with the silver oxide in a well-known manner, for example, as described in U. S. patent 1,901,578 to Bainbridge. The silver oxide is reduced, forming a layer consisting of a mixture of silver particles, caesium particles and caesium oxide particles, on which is deposited a thin caesium layer. If desired, the excess of caesium may either be removed from the envelope or bound therein in a conventional manner.

The wire I sealed into the wall constitutes the electric conductor for the photo-electric electrode or photo-cathode thus formed. The cathode is made thin enough to be transparent to light so that the light rays impingin on the outside of the envelope can pass thru to the inner side of the cathode. Therefore, it is not necessary to provide a window in the cathode.

To avoid the formation of a conductive layer on the stem, it may be shielded by means of a screen in the course of manufacture.

In the modification shown in Figure 2, the photo-electric electrode of the tube is formed on .the semi-cylindrical body or electrode 9 which acts as a cathode, the current strength used having such a value that the hollow side of the body 9 is coated with a silver oxide layer which is subsequently subjected in the manner set out above to the action of the electropositive metal.

The electrode 8 which receives a positive voltage with respect to the electrode III when the silver oxide layer is formed has with a normal use of the tube a negative potential with respect to the electrode l0, which acts as the anode.

While I have indicated the preferred embodiments of my invention of which I am now aware and have also indicated only onespeciilc application for which my invention may be employed, it will be apparent that my invention is by no means limited to the exact forms illustrated or the use indicated, but that many variations may be made in the particular structure used and the purpose for which it is employed without departing-from the scope of my invention as set forth in the appended claims.

What I claim as new is:-

l. The method of manufacturing a photo-cathode for a light sensitive electron discharge device having an envelope in which a metal oxide layer is to be formed and subjected to the action of an electropositive metal, which includes posi-.. tioning the metal from which the oxide layer is to be formed within the envelope out of contact with a base within the envelope on which the metal oxide layer is to be formed, introducing an atmosphere containing oxygen within said envelope, establishing a controlled electric discharge between said metal as a cathode and another electrode as an anode within the envelope, so that a layer of the oxide ofv said metal is deposited on said base, removing the atmosphere containing oxygenfroni the interior of the envelope and introducing an alkaline metal within the envelope to reduce the metal oxide to form a light sensitive electrodeon the interior of the envelope. a v

2. The method of manufacturing a photo-cathode in a light sensitive electron discharge device having an envelope of glass on which a metal oxide layer is to be formed and subjected to the action of an electropostive metal, including positioning within the envelope and out of contact with the envelope the metal from which the oxide layer is to be formed, introducing within the envelope and atmosphere containing oxygen, establishing a dischargev between said metal and an auxiliary electrode within said envelope, said metal serving as a cathode andsaid auxiliary electrode asan anode during said discharge, and controlling the strength of the discharge so that a metal oxide layer thin enough to be transparent to light is formed on the inside of the envelope, removing the atmosphere containing oxygen from the interior of theenvelope and introducing an alkaline metal within the envelope to reduce the metal oxide to form a light sensitive electrode on the interior of the envelope.

3. The method of manufacturing a photo-cathode in a light sensitive electron discharge device having an envelope containing a pair of parallel electrodes,- one of the electrodes including a silvered molybdenum rod and the other a semicylindrical copper body, including exhausting the envelope, introducing oxygen into said envelope, establishing a discharge between the electrodes, the silvered molybdenum rod acting as a cathode and the copper body as an anode during the discharge to form a silver oxide layer on the copper body, exhausting said oxygen from said envelope and subjecting the resulting oxide layer on said copper body to the action of an electropositive metal to form a light sensitive photo-cathode.

mam CORNELIB 'rEvEs. 

